Effective treatment of craniofacial pain requires knowledge of the relationships between neural structures, neurotransmitters and receptors that may be therapeutic targets. The physiological basis for these relationships is grounded in the anatomical arrangements between primary afferents, descending projections, interneurons, and efferent projection neurons in the trigeminal dorsal horn. The long-term goal of this work is to understand the anatomical relationship between neuronal elements that mediate or modulate nociception in the trigeminal dorsal horn. The proposed studies use corneal afferents as a nociceptive marker and will examine the relationships between these afferents and other neurons, including interneurons, projection neurons, and descending inputs, in the trigeminal dorsal horn. Studies in Aim 1 will determine if corneal afferents target inhibitory interneurons, trigeminothalamic or trigeminopontine projection neurons; thus defining the basic connectivity of these afferents. Corneal afferents project to two distinct regions of trigeminal dorsal horn, and these regions are thought to mediate functionally distinct components of corneal nociception. Preliminary data demonstrate that the neurotransmitter and transporter content of corneal afferent projections to these two regions are distinct. In Aim 2, differences in the expression of neuronal markers will be examined in corneal afferents, including various putative sensory transduction molecules. Additional studies will determine if select populations of corneal afferents target specific populations of target neurons defined in Aim 1. In Aim 3, the targets of descending medullary projections will be examined to determine if these neurons converge on similar neurons with corneal afferents. Together these studies will provide a more specific understanding of the anatomical relationships between corneal afferents and descending afferent projections, inhibitory interneurons and efferent projection neurons within the trigeminal dorsal horn. These results will provide information about the anatomical substrates for ocular pain and potential mechanisms for both neuronal plasticity and therapeutic targets in the trigeminal dorsal horn. PUBLIC HEALTH RELEVANCE Orofacial pain represents a significant public health concern, afflicting more than 10% of adults, and 50% of elderly adults. Pain arising from corneal injury is on the rise due to the increased number of Americans undergoing refractive eye surgery, including PRK and LASIK (Belmonte, 2004, 2007). Other sources of ocular pain include dry eye, uveitis, and contact lens use. The studies in this proposal are designed to understand the neural pathways involved in transmitting pain from the cornea to the brain.